Non-aqueous secondary batteries, such as those represented by lithium ion secondary batteries, have been widely used as power supplies for portable electronic devices such as laptop computers, mobile phones, digital cameras, or camcorders.
In recent years, with the reduction in size and weight of portable electronic devices, reduction in weight of an outer casing of a non-aqueous electrolyte battery has been realized. As an outer casing material, a can made of aluminum has been developed in place of a can made of stainless steal, and further, a pack made of aluminum laminate film has been developed in place of a can made of metal.
However, since packs made of aluminum laminate film are soft, in a battery (a soft pack battery) using the pack as the outer casing, a space is easily formed between the electrode and separator, due to impact from the outside or expansion⋅shrinkage of the electrode accompanying charging and discharging, which may result in a reduction in cycle life.
In order to address the above problem, techniques for enhancing adhesion between an electrode and a separator have been proposed.
As one of such techniques, a separator that an adhesive porous layer made of a polyvinylidene fluoride resin is formed on a polyolefin microporous membrane has been known (see, for example, Patent Documents 1 to 4). When this separator is superposed on an electrode and heat-pressed, the separator may favorably adhere to the electrode via the adhesive porous layer, and thus, the cycle life of a battery may be improved.
Generally, in a process of producing a soft pack battery using a separator having the above adhesive porous layer, electrodes and a separator are stacked together and rolled up, to prepare a battery element. This process of preparing a battery element is similar to the process of preparing a conventional battery element in the process of producing a battery having a metal can outer casing. Thus, it is advantageous in that it is not necessary to greatly change the conventional process of preparing a battery element, in the case of applying a separator having the above adhesive porous layer to a soft pack battery.
In view of the background described above, a separator that an adhesive porous layer made of a polyvinylidene fluoride resin is formed on a polyolefin microporous membrane is suitable for a soft pack battery, and various technical proposals have been made, focusing on further improvement in performance.
For example, the separator disclosed in Patent Document 1 focuses on the porous structure and thickness of an adhesive porous layer, from the viewpoint of achieving both the adhesion to electrodes and ion permeability. In the separators disclosed in Patent Documents 2 to 4, an adhesive porous layer is formed by using two kinds of polyvinylidene fluoride resins in combination, from the viewpoint of adhesion to electrodes.
Patent Document 1: Japanese Patent No. 4127989
Patent Document 2: Japanese Patent No. 4490055
Patent Document 3: Japanese Patent No. 4109522
Patent Document 4: Japanese Patent No. 4414165